The physical properties of a crystalline polymer product such as a synthetic fiber, a resin film, a resin sheet, or the like are intensely influenced by the internal structure thereof (fine structure of a crystalline polymer), and the internal structure is relatively easily altered by drawing or heat treatment. And, in many cases, a drawn product has physical properties more desirable in practical use than a non-drawn product, and at a higher draw ratio, a drawn product excellent in physical properties such as strength and Young's modulus. Generally, drawing treatment is therefore carried out for obtaining crystalline polymer products, particularly a synthetic fiber, a resin film and a resin sheet. Further, a drawn product is heat-treated as required.
As a drawing method for obtaining crystalline polymer products, various methods are known. For example, when a synthetic fiber is produced, there is applied a method of drawing such as contact heat drawing with a metal hot roll or a metal hot plate or non-contact heat drawing with hot water, steam having a pressure of approximately atmospheric pressure to 2 kg/cm.sup.2 or far infrared.
Meanwhile, a change in the fine structure of a crystalline polymer is influenced by drawing conditions to a great extent, and as a result, the physical properties of a crystalline polymer product are also influenced by the drawing conditions to a great extent, while failures such as draw breaking is caused when drawing is forcibly carried out.
For example, in a polypropylene (to be abbreviated as "PP" hereinafter) fiber, the fiber strength thereof is improved when a non-drawn yarn is drawn at a temperature which is lower than the melting point of the resin (PP) but is as high as possible, at a low deformation rate at a higher draw ratio. When attempts are made to draw the fiber at a high deformation rate at a high draw ratio, draw breaking easily takes place. The fiber strength of industrially (commercially) producible PP fibers, i.e., the fiber strength of PP fibers that can be produced at a rate of approximately 50 m/minute or higher is approximately 10 g/d (e.g., see Japanese Patent 2,537,313).
However, when drawing is carried out at a very low deformation rate without considering productivity, there can be obtained a PP fiber having a higher fiber strength. For example, a Japanese periodical, "Kobunshi Ronbunshu" (Vol. 54, No. 5, May, 1997), pages 351 to 358 describes a PP fiber having a fiber strength of approximately 13.4 g/d, produced by a continuous zone drawing method. When the above PP fiber is obtained, however, the feed rate of a fiber in a continuous zone drawing step is as small as 0.5 m/minute.
As described above, a change in the fine structure of a crystalline polymer is greatly influenced by drawing conditions, and as a result, the physical properties of drawn product formed of the crystalline polymer are greatly influenced by the drawing conditions. When forcible drawing is attempted, there are caused failures such as draw breaking. The physical property values of a drawn product formed of a crystalline polymer, which can be obtained by a conventional drawing method, have upper limits depending upon the material quality of the drawn product formed of the crystalline polymer.
However, products formed of crystalline polymers are used in various fields, and with an increase in demands thereof, products formed of the crystalline polymers are constantly required to be improved in physical properties.